Plastic orthodontic brackets (e.g., polycarbonate) are well known in the art and provide certain advantages over metal and ceramic brackets since they are relatively light weight and aesthetically pleasing. Plastic orthodontic brackets are typically bonded directly to the enamel surfaces of teeth with an acrylic dental bonding cement, generally of the light cure or A/B dual-component, self-curing type. The bond strengths formed between acrylic cements and plastic brackets is not always sufficient to ensure that the bond will not break during the course of orthodontic treatment, either due to the forces applied by orthodontic archwires or by other external forces such as mastication.
One generally accepted approach for improving the bond strength of plastic brackets to clinically acceptable levels is to use an adhesive primer at chairside, just prior to applying the acrylic adhesive, and before bonding. This approach adds an additional step in the bonding procedure and presents a variety of drawbacks. These drawbacks include the additional chairside time required of the clinician; the added expense since the primer is a separate item which is relatively costly; and a limited working time window, on the order of minutes, for successful retention of the bracket by the bonding adhesive. Furthermore, the use of primers introduces additional complications in the bonding procedure such as the difficulty in adhesive placement and/or dilution of the adhesive paste to a consistency which causes excessive bracket drift, and the inconsistencies in bond strength which are directly related to the time/temperature dependence of diffusion of the primer into the plastic substrate. That is, since the primer acts on each bracket for a different length of time, the ultimate bond strength of each bracket may be different. Moreover, non-equivalent results are achieved depending on the type of adhesive, i.e., mix, no-mix, light-cure, etc., and adhesive primers may present an unpleasant, pungent odor and are oftentimes irritating to the patient's skin. Lastly, some of the more effective adhesive primers are being restricted in chairside use due to their suspected carcinogenic contents.
Another approach directed at enhancing the bond strength of plastic brackets with acrylic adhesives is to employ special adhesives which contain additional compounds having a high affinity to the plastic bracket substrates. Examples of this approach are found in U.S. Pat. Nos. 4,134,929, 4,228,062 and 4,340,529.
The present invention is believed to overcome the various drawbacks discussed above with respect to the known techniques for enhancing the bonding of plastic orthodontic brackets to enamel tooth surfaces.